Powder particles which may be used for powder coating applications such as powder painting are advantageous in that they eliminate the disadvantages associated with the use of solvents in liquid paint systems, as for example, the liquid paint system disclosed in U.S. Pat. No. 2,857,354. Powder paint compositions are curable by heat in such a way that little, if any, volatile material is vaporized or evaporated into the environment. This, of course, is substantially different from liquid paint systems where the liquid paint vehicle is volatilized during the drying of the paint.
Processes for preparing powders for various applications are disclosed, for example, in U.S. Pat. Nos. 3,737,401, 3,730,930, and 3,770,848.
It is known that metal particles or flake such as aluminum flake tend to be shear degradable. In the production of shear degradable particle-containing powder particles in general, and in the production of metal particle-containing powder particles in particular, there has been a problem in the past with degradation of the shear degradable ingredient. Since the shear degradable ingredient may be used primarily for decorative purposes (e.g., the use of aluminum flake in automobile top coats, where it is necessary that the finish have a metallic look), any degradation of the shear degradable ingredient presents a serious aesthetic problem.
It is known to produce metal particle-containing powder particles by grinding metal particle-containing resin sheets to form pellets. However, this grinding operation tends to degrade the metal particles contained within the resin.
A prior art method for preparing metal particle-containing powder particles is disclosed, for example, in U.S. Pat. No. 3,739,049. A particularly efficacious method is disclosed in U.S. Pat. No. 3,803,111 wherein such powder particles are prepared by spray-drying acrylic polymer solutions.
The search has continued for improved processes for preparing shear degradable particle-containing resin powders wherein the shear degradable particles remain substantially undegraded within the resin. This invention was made as a result of that search.